An efficient after treatment technique is driven by the need to maintain strict emission norms for heavy-duty and medium-duty ground vehicles. SCR being an advanced active emission technology system for diesel engine, is one of the most cost-effective and fuel-efficient technologies available for complying with the stringent NOx emission legislations. The design of the SCR system involves catalyst selection, complex controller development like urea dosing strategy and the interaction between engine setup and after treatment system. For this purpose, the SCR model must be computationally efficient to evaluate the complete efficiency along with to take care for the NH3 slip also. The SCR model was prepared with respect to SCR inlet temperature and ratio of NOx and ammonia to study the behavior of NOx conversion efficiency keeping consideration of NH3 slip also required for optimizing the calibration. An investigation was done into the urea utilization and NOx slip over an SCR catalyst was completed for the purposes of determining the proper calibration for NOx reduction over a downstream SCR. Experiments were conducted in an engine dynamometer on the 5.64 lts. diesel engine and optimization was done to get the optimum NOx conversion efficiency with the temperature based model.